In memory applications in the semiconductor industries, such as DRAM and 2D NAND, plasma etching removes silicon-containing layers, such as SiO or SiN layers, from semiconductor substrates. For novel memory applications, such as 3D NAND (US 2011/0180941), etching of stacks of multiple SiO/SiN or SiO/poly-Si layers is critical. Preferably, the etchant has high selectivity between the mask and layers being etched. Furthermore, the etchant preferably etches the structure such that the vertical profile is straight, with no bowing. The 3D NAND stack may include other silicon containing layers.
Traditionally, plasma etching is carried out using a plasma source which generates active species from a gas source (such as hydrogen-, oxygen-, or fluorine-containing gases). The active species then react with the Si-containing layers to form volatile species. The volatile species are removed by low pressure in the reactor, which is maintained by a vacuum pump. Preferably, the mask material is not etched by the active species. The mask material may comprise one of the following: photoresist, amorphous carbon (a-C), polysilicon (polySi), metals, or other hard masks that do not etch.
Traditional etch gases include cC4F8 (Octafluorocyclobutane), C4F6 (Hexafluoro-1,3-butadiene), CF4, CH2F2, CH3F, and/or CHF3. These etch gases may also form polymers during etching. The polymer acts as protection or passivation layers on the sidewalls of the pattern etch structure. This polymer passivation layer prevents the ions and radicals from etching the sidewalls, which may cause non-vertical structures, bowing, and change of dimensions. It is well known in the art that selectivity and polymer deposition rate increases as the ratio of C:F increases (i.e., C4F6>C4F8>CF4). See, e.g., U.S. Pat. No. 6,387,287 to Hung et al.
Traditional etch chemistries may not provide the high aspect ratio (>20:1) necessary in new applications due at least to insufficient polymer deposition on side walls during the plasma etching process. Additionally, CxFy polymers on sidewalls are susceptible to etching. As a result, the etched patterns may not be vertical and structures may show bowing, change in dimensions, and/or pattern collapse.
Bowing may result from sidewall etching of the mask layer, which is often an amorphous carbon material. Amorphous carbon materials may be etched by oxygen radicals in the plasma which may cause increased opening of the mask and result in the bow-like, or angled/curved, etch structure.
Sulfur gases like COS (carbonyl sulfide) and SO2 (sulfur dioxide) have been used in the past in combination with oxygen plasma to etch the amorphous carbon layer in the pattern etch process. The sulfur may provide a passivation layer on the amorphous carbon to help protect the surface from oxygen radicals and therefore help to prevent the bow-like structures. For example, Kim et al (J. Vac. Sci. Technol. A 31 (2), March/April 2013) disclose that a 50 nm amorphous carbon hole etched in a gas mixture of O2 and 5% COS produce a more anisotropic etch profile and improved the top/bottom opening ratio by about 37% as compared to those etched without COS.
Rusu et al (U.S. Pat. No. 7,645,707) describe the process of etching a dielectric layer using an etchant gas comprising a fluorine component, O2, and a sulfur component gas The sulfur component gas is preferably H2S, COS or CS2.
Yanagida (U.S. Pat. No. 5,376,234) discloses a dry etching method wherein one compound selected from mercaptan, thioether and disulfide each having a fluorocarbon side chain is used as a main component of the etching gas. Examples in which C2F6S2 is used to etch a SiO2 interlayer insulation film are provided.
US2003/0019841 to Behr et al. discloses the addition of perfluorochemicals, such as CF3SF5, to a cleaning or etching gas.
KR10-2001/010568 to Samsung Electronics Co. Ltd. discloses dry etching of oxide films using sulfur-containing fluorocarbon gases, such as C4F8S, C3F6S, and C3F6S2.
Molecules containing thiocarbonyl groups (>C═S) and fluorine atoms have also been proposed for different etch processes. See, e.g., JP06-151384, JP06-258815, and JP07-211694 to Sony Corp.
A need remains for new etch gas compositions for use in plasma applications.